A distributive lighting system for automotive exterior lighting generally includes a high intensity discharge (HID) light source, a large-core optical fiber light pipe or fiber bundle, and a collection system to couple light into the optical fiber. The system may utilize a single fiber, or multiple fibers may be branched from the same light source. The minimum photometric purpose of an automotive exterior lighting lamp package, the optical system connected to the output end of a distributive lighting system, is the projection of visible light at angles and luminous intensities (candela) that comply with automotive exterior lighting photometry requirements.
A distributive lighting system coupled to an efficient low beam head-lamp package has advantages over conventional tungsten-halogen head lamp packages. Typical distributive lighting systems are capable of delivering 796 to 1505 lumens to a lamp package while consuming only 35 W of electrical power per lamp. A typical sealed beam or composite head lamp supplies 350 to 450 lumens to the beam pattern region using a 55 W tungsten-halogen source. A lamp package optical efficiency greater than 40% would provide increased optical performance with lower power consumption than current tungsten-halogen lamps.
Incremental improvements in automotive lighting have increased demand for improved durability, safety, and styling of exterior lighting systems. The quality of head lamp performance beyond government requirements is further specified by both OEMs and their customers. Photometry tests required in the Federal Motor Vehicle Safety Standards (FMVSS 108) and night driving experience provide the following criteria for low beam head lamps:
1. Down-the-road lighting PA1 2. Glare light control above the horizontal PA1 3. Horizontal beam spread PA1 4. Foreground illumination PA1 5. Beam pattern uniformity PA1 1. In general, the more luminous flux directed down-the-road, the better the down-the-road illumination. PA1 2. In general, the less luminous flux directed above the horizontal, the better the glare light control. PA1 3. Light must be spread at least enough to meet the 850 candela minimum luminous intensity specified at the 2D-15L and 2D-15R test points. In addition, wider horizontal spread in the beam pattern is generally desirable. PA1 4. Light must be directed below horizontal at least enough to provide uniform lighting at the driver's perceived area directly in front of the vehicle. PA1 5. Uniformity of the beam pattern is generally considered to be a positive, although subjective, performance characteristic by the customer. PA1 1. Controlling the flux distribution from the optical fiber. PA1 2. Controlling the angular distribution emitted from the lamp package.
These criteria may be extended into the following design guidelines:
The key to designing any low beam head lamp which meets legal and customer photometric performance standards lies chiefly in meeting these five design guidelines.
The design approaches to meet the low beam guidelines may be classified according to the light collection method. For example, one may use a reflector, an aspheric lens, or a plastic light guide. Projector lamp approaches have an advantages over reflector approaches in that they are feed forward systems. Feed forward systems do not require two reflective surfaces and thereby eliminate lamp geometry concerns of shadowing due to mirror placement. Also, the projector lens approaches yield superior glare light control, and better beam pattern uniformity. Thus, while the discussion below refers primarily to low beam lamp packages which utilize a projector lens as the primary light collecting component, it should be understood that other approaches are possible.
Performance of an optical system generally depends on (1) the numerical aperture of the output end of the distributive lighting system, which is the numerical aperture of the input to the lamp package, (2) the diameter of the optical fiber core, (3) the total luminous flux emitted from the fiber, and (4) the lumen density (lumens/mm.sup.2) across the output face of the fiber. Each of these will be discussed in turn.
The numerical aperture of the light emitted from a straight optical fiber is: EQU NA=n.sub.o sin.theta..sub.max =(n.sub.f.sup.2 -n.sub.c.sup.2).sup.1/2
where n.sub.o is the index of refraction of air, -.theta..sub.max is the maximum acceptance angle of the optical fiber-core cladding interface, n.sub.f is the index of refraction of the cladding, and n.sub.c is the index of refraction of the optical fiber core. If n.sub.o is taken to be equal to 1.0, then the NA is simply equal to the sine of the maximum acceptance angle of the optical fiber.
In practice, calculating the NA based on indices of refraction does not always prove to be useful for two reasons. First, styling guidelines often require that the lamp package height be limited. Some balance must often be struck between the light collected by the aspheric lens in a lamp package and the maximum allowable height of the lamp package. Limiting the height of the aspheric lens may sacrifice a portion of the light emitted from the fiber since it is not always possible to increase the numerical aperture of the lamp optical system. Second, the angular distribution of the luminous flux coupled into the optical fiber provides more light at smaller angles that at larger angles. Therefore, there may be very little light available at the larger angles for the lamp package. Light at the larger angles may also be limited by bends in the fiber, which force the rays at larger angles to escape from the fiber core.
Establishing the numerical aperture for a lamp design that both satisfies the package height requirements and sacrifices only a small portion of the available luminous flux is often necessary. In one example where the input pattern was Gaussian, only twenty percent of the available luminous flux was lost when the numerical aperture was reduced from 0.66 to 0.45. This allowed the required lens diameter to be reduced from 79 mm to 45 mm.
A small numerical aperture for light emitted from the fiber is desirable for designs that require lenses to collect the light. The smaller the numerical aperture, the smaller is the lens diameter required to capture available light. Small numerical apertures allow for longer lens focal lengths and, therefore, larger manufacturing tolerances. Larger numerical apertures have the advantage of shorter focal lengths, which decreases lens lamp package depth. Clearly, compromises must be made in determining the lens design NA and focal length of the optical system.
The lumen distribution across the output surface of the fiber core is critical to the optical design. The output surface of the optical fiber may be described by the source distribution function S(x,y), which describes the lumens per mm.sup.2 at x,y coordinates on the fiber output surface. A source lumen distribution located at the focal distance from an aspheric lens and whose luminous flux is completely collected by an aspheric lens is transformed into an angular intensity distribution -1(.theta.,.phi.) as: EQU I(.theta.,.phi.)=T{S(x,y)} EQU x=f tan.theta. EQU y=f tan.phi.
The transformation function T may be approximated by the following substitutions:
where "f" is the focal length of the aspheric lens and a constant for unit conversion is also applied.
The goal of designing an optical configuration which projects an angular intensity distribution in the far field that meets our evaluation criteria can be reduced to achieving the desired -1(.theta.,.phi.). As can be seen from the above formulas, the choice of S(x,y) and f determines -1(.theta.,.phi.). Therefore, the degree to which we are able to control S(x,y) is the degree to which we are able to control the hot spot intensity, down the road light, glare light, beam pattern uniformity, and beam spread angles.
Several techniques are known for collecting light from an optical fiber and forming it into a desired pattern. One such technique is to reflect light emerging from the fiber off two flat mirrors in a V-configuration into a standard head lamp parabolic reflector. The mirror and reflector collect and quasi-collimate the light from the fiber. An array of small lenslets forms a cover lens, which controls the angular output from the lamp package to form a legal beam pattern.
Another approach utilizes an optical fiber and a lamp device with no additional optics to form the beam pattern. Such a device is shown, for example, in U.S. Pat. No. 5,436,806 (Kato) and includes a lamp device that has a light bending and conducting path and light conducting, path lenses that produce the desired light distribution.
Yet another arrangement includes an optical structure in the form of at least one step affixed to the end of the output end of a light conductor. The step provides an output face located at the focal point of a projector lens, while the remaining parts of the light conductor or the optical structure are displaced from the focal point and, thus, spread by the projector lens because they are out of focus. An optical structure having more than one step may be used to generate light sources at various distances from the focal plane of the projection lens, but this optical structure does not include individual light conducting paths and cannot truly redistribute the light pattern. An example of such a system is shown in U.S. Pat. No. 5,257,168 (Davenport).
Still another system (e.g., that shown in U.S. Pat. No. 5,184,882 to Davenport) employs a plurality of elongated light conductors to conduct light from a central source of light to a projection lens, which, in turn, projects the desired light pattern on the road. This system is complex and expensive because it requires a bundle of several long fibers that extend from the central light source to the projection lens. Because the long fibers are specifically designed for one particular application, e.g., a vehicle headlamp, this system does not permit use of off-the-shelf or easily interchanged parts.
Other known systems are multi-channel systems that use a plurality of fibers, each of which is associated with an individual projection lens element